Here is the full list of all 23 Mail Time achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
deliver meals instead of mail!
you did it!! kinda... not really.... BUT you did do other things!
glide for 15 minutes!
sample all the rare mushrooms!
meet everyone in the neighbourhood!
find 40 trinkets!
deliver a fan's letter to their idol!
complete 50 deliveries!
prevent a crime from taking place!!
deliver an anonymous love letter <3
complete 25 deliveries!
do your part & clean up the forest!
find 20 trinkets!
glide for 5 minutes!
complete 10 deliveries!
deliver a letter in under 30 seconds!
file an official letter of complaint >:(
take over 15 minutes to deliver a letter!
find all rare coins!
glide for 60 seconds!
find 5 trinkets!
get to know your neighbourhood
Deliver your very first letter!